The invention relates generally to an apparatus for cooling a heated surface and, more particularly, to a heat sink with microchannel cooling for semiconductor power devices.
The development of higher-density power electronics has made it increasingly more difficult to cool power semiconductor devices. With modern silicon-based power devices capable of dissipating up to 500 W/cm2, there is a need for improved thermal management solutions. When device temperatures are limited to 50 K increases, natural and forced-air cooling schemes can only handle heat fluxes up to about one (1) W/cm2. Conventional liquid cooling plates can achieve heat fluxes on the order of a twenty (20) W/cm2. Heat pipes, impingement sprays, and liquid boiling are capable of larger heat fluxes, but these techniques can lead to manufacturing difficulties and high cost.
An additional problem encountered in conventional cooling of high heat flux power devices is non-uniform temperature distribution across the heated surface. This is due to the non-uniform cooling channel structure, as well as the temperature rise of the cooling fluid as it flows through long channels parallel to the heated surface.
One promising technology for high performance thermal management is microchannel cooling. In the 1980's, it was demonstrated as an effective means of cooling silicon integrated circuits, with designs demonstrating heat fluxes of up to 1000 W/cm2 and surface temperature rise below 100° C.
Other micro-channel heat sink designs also demonstrate similar thermal performance. Published US Patent Application No. 20030066634A1, Valenzuela et al., entitled “Heat Exchanger,” provides an improvement relative to conventional liquid cooling designs. The device of Valenzuela et al. involves a normal-flow microchannel heat sink, which combines the benefits of sub-millimeter scaled channels with vertical flow towards the heated surface. In most microchannel designs, the increased heat transfer is offset by increased pressure losses in the small-scale channels, which results in increased pumping needs. By altering the flow passage geometry to produce flow normal to the heated surface rather than parallel to it, shorter channels are possible, so the pressure loss is not severe. However, the manufacture of this heat sink is somewhat difficult, as micro-channels and larger fluid supply/return passages must be first machined on individual copper laminations, which are then assembled in a stack.
U.S. Pat. No. 6,014,312, Schultz-Harder et al., entitled “Cooler or heat sink for electrical components or circuits and an electrical circuit with this heat sink,” also provides an improvement relative to conventional microchannel liquid cooling designs. The device of Schultz-Harder et al. also consists of a stack of copper laminations with overlapping, but slightly offset, millimeter-sized holes, resembling a stack of sliced Swiss cheese. The fluid flow is parallel to the heated surface. The holes introduce additional turbulence in the liquid, enhancing the heat transfer, but also resulting in a higher pressure drop. Device manufacturing requires precise alignment of the laminations before they are bonded together. Based on published thermal performance results, the design of Schultz-Harder et al. is less effective than that of Valenzuela et al, but it is still better than conventional heat sinks.
U.S. Pat. No. 5,692,558, Hamilton et al., entitled “Microchannel cooling using aviation fuels for airborne electronics,” describes a micro-channel heat sink for cooling of semiconductor devices with aircraft fuel. The channels are built directly in the device's semiconductor substrate to reduce junction-to-fluid thermal resistance. U.S. Pat. No. 5,998,240, Hamilton et al., entitled “Method of extracting heat from a semiconductor body and forming microchannels therein,” (Hamilton II) also discloses forming of micro-channels directly in the semiconductor substrate of the radio-frequency (RF) power device. Additionally, Hamilton II discloses a full three-phase inverter with direct cooling of switching power devices (IGBT's and diodes). Although implementation of microchannels directly in the semiconductor device's substrate may be suitable for low-voltage RF applications, it is not practical for the switching power inverter applications. The IGBT and diode power devices for switching inverter applications have vertical structure, and the bottom of the semiconductor substrate is a power terminal (collector/cathode) that connects to other circuit components. The collector/cathode terminal of the switching power device alternatively conducts substantial current (as much as 150A) and blocks voltage that could exceed thousand volts. Implementation of the micro-channels at the bottom of the device would be impractical for several reasons. The implementation would interfere with the electrical interconnect to other circuit components, the implementation would expose cooling fluid to high voltage and put severe restrictions on suitable fluids and/or necessitate de-ionization equipment, and the implementation would exacerbate electromagnetic interference (EMI) problems due to cooling fluid becoming conduction path for high frequency currents.
U.S. Pat. No. 5,727,618, Mundinger et al., entitled “Modular microchannel heat exchanger,” describes a micro-channel heat sink for cooling a high heat flux laser diode array. The heat sink features micro-channels formed in multiple copper sheets, and practical implementation of the micro-channels requires vertical stacking and brazing of multiple copper sheets to achieve desired (tall) aspect ratio. It also requires stacking and bonding of multiple copper plates with etched geometries for vertical manifolding of cooling fluid from inlet pipe up to the heated surface and back down to the fluid outlet. There are several disadvantages of this design. Manufacturing of the heat sink requires etching, metallization, stacking, precise alignment, and bonding of a number of copper foils in order to achieve sufficient channel depth. The same is true for stacked copper plates that form the inlet and outlet manifolds. Scalability to larger heat sink sizes is limited by fluid manifolding in a vertical direction and it may lead to excessive heat sink thickness. Finally, Mundinger fails to address implementation of electrical isolation between the semiconductor power devices and the cooling fluid.
U.S. Pat. No. 6,131,650, North et al. entitled “Fluid cooled single phase heat sink,” describes a heat sink that places a pad of porous metal in contact with the heated surface. One drawback of this approach is that thermal performance of the porous metal pad is less effective.
It would therefore be desirable to provide an apparatus for cooling a heated surface with improved thermal performance, relatively simple assembly to reduce manufacturing cost, and scalability for accommodating small and large power devices as well as different numbers of power devices. In addition, it would be desirable for the apparatus to provide electrical isolation between high power devices and the coolant.